The study of the geology of the internal structure of the shallow portion of the earth's crust, within 100 meters of the surface, is often conducted using seismic waves. Seismic waves include shear-horizontal (SH), shear-vertical (SV) and longitudinal (P) waves which may be generated using devices which propel an impact mass against a rigid earth contacting target. Measurements of seismic waves may provide detailed information concerning both rock properties and depth.
Conventional devices generate seismic shear waves using a horizontal propulsion of an impact mass. The propulsion force is often administered manually by using a hammer to hit against a weighted plank or metal fixture. For certain applications, these devices have not always been satisfactory, because the amount of force they are capable of generating is limited. In particular, at some landfill sites, the filling material may drastically absorb seismic energy to such an extent that the seismic waves produced by manual hammer devices cannot penetrate to the base of the fill and, thus, the thickness of the fill cannot be seismically determined.
Alternatively, many new designs for seismic wave sources employ pneumatically driven pistons which alternately and oppositely strike contained anvils. These devices are advantageous because they are more powerful than manual hammer devices. However, the problems which occur with these pneumatic devices are those inherent in any closed pneumatic system such as condensation within the cylinders and entrained water in the air supply.
Others have attempted to overcome the problems associated with both the manual hammer and pneumatic devices. Matsuyama, U.S. Pat. No. 5,079,463, for example, discloses the use of a flywheel positioned for rotation in the horizontal plane to generate seismic waves. The flywheel is rotated at a predetermined speed and braked continuously or intermittently to generate a reaction force. However the device is only capable of generating SH waves.
Meissner, et al., "Shear-Wave Exploration: Geophysical Development Series", 1 Society of Exploration Geophysicists, 1986, disclose a pneumatically driven shear-wave source. The device provides a more powerful impact than that of a hammer and may be transported to a testing site using a trailer. However, the device requires the use of separate P and S sources. Moreover, the device is constructed such that it is constrained to operate only at right angles to the vehicle on which it is mounted.
Airhart, U.S. Pat. No. 4,660,674 discloses a device for generating seismic waves which can be controlled so as to deliver energy to a point on the earth's surface along a plurality of selected slant paths. The device includes a hollow cylindrical housing and an impact mass slideably interfitted within the housing. The device is capable of generating both SH and SV waves. However, it tends to be somewhat cumbersome since the entire housing must be rotated into alignment with a preset slant path.